Process and apparatus for the production of a low-shrinkage aliphatic polyamide yarn, and low-shrinkage yarn

ABSTRACT

An apparatus for production of a low-shrinkage aliphatic polyamide fibre, in which polyamide is extruded through a spinneret to form filaments, then cooled and combined to form at least one yarn. The at least one yarn is subjected to drawing between the spinneret and a pair of inlet rolls, then in a further multi-stage drawing step is subjected to 4-fold to 6-fold drawing by pairs of draw rolls. The pairs of draw rolls successively heat the yarn and at least the last pair of draw rolls has a temperature of 5° C. to 20° C. below the melting point of the yarn. The yarn is relaxed by from 6% to 10% in a subsequent at least three-stage relaxation zone and is kept in a temperature range of 5° C. to 15° C. below the melting point of the yarn, and is subsequently wound up on a reel device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/537,866, filed Jun. 19, 2017, which is a U.S. National Stage ofInternational Patent Application No. PCT/EP2015/002455 filed Dec. 5,2015, designating the United States and claiming benefit of GermanPatent Application No. 10 2014 119 184.6 filed Dec. 19, 2014, the entirecontents of the foregoing applications being incorporated herein byreference.

BACKGROUND OF THE INVENTION

The invention relates to a process and an apparatus for the productionof a low-shrinkage aliphatic polyamide yarn, in which polyamide isextruded through a spinneret to form filaments, then cooled and combinedto form at least one yarn, the yarn being subjected to drawing in amulti-stage process and then relaxed. The invention further relates to alow-shrinkage aliphatic polyamide yarn.

High-tenacity polyamide yarns are suitable for a large number oftechnical purposes, such as, for example, for use as reinforcement intyres or conveyor belts, for use in plastics-coated materials and incables or for the production of airbags. In order to produce such yarnseconomically, the product properties should have a combination of hightenacity and low shrinkage on heating. Especially desirable propertiesare a tenacity of at least 80 cN/tex and a hot air shrinkage of lessthan about 5.0%. That combination of properties is difficult to achieve,especially in a combined spinning and drawing process at high speed inindustrial mass production.

The starting material used for the production of airbag yarn is usuallypolyamide 6.6 or PA6.6, an aliphatic polyamide composed of monomericunits derived from hexamethylenediamine and adipic acid, which has amelting point of around 260° C.

The use of the significantly more readily available polyamide 6, apolyamide composed of monomeric units derived from caprolactam, asstarting material for an airbag yarn had the disadvantage of arelatively low melting point, meaning that on deployment of the airbagthe yarn could more easily be destroyed by the flying sparks. That wasthe case in the past, because high-temperature inflation systems wereused. Since the deployment of airbags nowadays takes place at lowertemperatures, the use of polyamide 6 is now a possibility.

EP 2 205 780 B1 describes a spin-draw process for the production ofairbag yarn from PA 6.6, wherein a multifilament yarn is created frommolten polyamide by means of a spinneret, which multifilament yarn isfirst subjected to multi-stage drawing at a temperature of 160° C. to245° C., then relaxed and subsequently wound up under tension. Thatprocess is not suitable for PA 6, because the holding temperature duringthe drawing is above the melting point of PA 6. The process is notsuitable for creating a yarn having suitable hot-air shrinkage from PA6.

EP 1 666 647 B1 describes a process for the production of airbag yarnfrom polyamide, wherein a multifilament yarn composed of polyamide 6.6is produced by means of melt-extrusion, the yarn is subjected to a highdegree of drawing at low temperature in a first step and to a low degreeof drawing at high temperature in a second step, then relaxed andsubsequently wound up. That process is likewise unsuitable forprocessing PA 6 on account of the high temperature, since it uses adrawing temperature of up to 250° C. and a relaxation temperature of upto 260° C. Furthermore, the production of airbag yarn from PA6.6 inaccordance with the above-mentioned processes is unsatisfactory, becausethe values for the tenacity and the hot-air shrinkage are subject to toomuch variation and do not yield the required values.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a process and anapparatus with which a polyamide can be reliably further processed toform a yarn in a spin-draw process so that it has a tenacity of at least80 cN/tex and a hot air shrinkage of a maximum of 6.5%. Hot-airshrinkage is hereby understood to be a value measured in accordance withASTM D4974-04 at 177° C. for 2 minutes,

The above and other objects are met by a process for the production of alow-shrinkage aliphatic polyamide yarn, in which the polyamide isextruded through a spinneret to form filaments, then cooled and combinedto form at least one yarn, wherein the at least one yarn undergoes firstdrawing between the spinneret and a pair of inlet rolls, then in afurther multi-stage drawing step is subjected to 4-fold to 6-folddrawing by means of pairs of draw rolls, the pairs of draw rollssuccessively heating the yarn and at least the last pair of draw rollshaving a temperature of 5° C. to 20° C. below the melting point of theyarn, the yarn being relaxed by from 4% to 10% in a subsequent at leastthree-stage relaxation zone and being kept in a temperature range of 5°C. to 15° C. below the melting point of the yarn, and subsequently beingwound up on a reel device.

According to the invention, a first multi-stage drawing step is followedby an at least three-stage relaxation step, so that the desired tenacityvalues are achievable even for a PA6 or a comparable polyamide. Inrespect of an airbag yarn composed of PA 6.6, this results in a stableproduction process in which the required values are reliably achieved.

In that process, the temperature in the last drawing stage is 5° C. to20° C. slightly below the melting point of the yarn. During the at leastthree-stage relaxation, the temperature is likewise kept just below themelting point of the yarn, namely 5° C. to 15° C., preferably 5° C. to8° C. below the melting point of the yarn.

Accordingly, the polymer chains (molecules) of the polyamide are able tostabilise, resulting in enhanced tenacity values unattainable hithertowith a yarn composed, for example, of PA6.

Due to the fact that the relaxation of the yarn between the third pairof draw rolls and at least two further pairs of draw rolls is effectedat a temperature slightly below the melting point, rapid and highconsolidation of the molecule chains of the polyamide is achieved.

In an embodiment, the subsequent relaxation between the last pair ofdraw rolls and the reel device also provides for sufficientstabilisation of the polymer chains. The utilisation of the reel devicefor relaxation of the yarn makes it possible to dispense with a furtherpair of draw rolls.

According to the invention the aliphatic polyamide comprises at least90% by weight, preferably at least 95% by weight, especially preferablyat least 98% by weight, monomeric units derived from caprolactam, ormonomeric units derived from hexamethylenediamine and adipic acid (AHsalt), or mixtures thereof. The weight percentage is given with respectto the total weight of polyamides. The polyamide may further containdyes, stabilisers etc.

Monomeric unit derived from caprolactam is also known by the chemicalformula (1):

—HN(CH₂)₅CO—  (1)

Monomeric unit derived from hexamethylene diamine and adipic acid isalso known by the chemical formula (2), and may also be derived from thesalt of hexamethylene diamine and adipic acid:

—HN(CH₂)₆NHCO(CH₂)₄CO—  (2)

The apparatus according to the invention for the production of alow-shrinkage aliphatic polyamide yarn comprises at least one spinneretby means of which polyamides are extruded to form filaments, then cooledand combined to form at least one yarn, there being arranged after thespinneret a first pair of inlet rolls for the first drawing of the yarn,which is followed by a second multi-stage drawing step, subsequentlyfollowed by an at least three-stage relaxation zone and a reel device.

Due to the fact that the drawing step is followed solely by successiverelaxation up to the reel device, the molecules are able to undergo alonger period of alignment and stabilisation. The integration of thereel device for relaxation of the yarn makes it possible to dispensewith a pair of draw rolls, with the result that the system can be moreadvantageously and compactly constructed.

In an embodiment, the multi-stage second drawing step comprises threepairs of heatable draw rolls which are in the form of duo rolls.

The problem is likewise solved by a thread composed of a low-shrinkagealiphatic polyamide fibre, produced from a polyamide which comprises acomposition of at least 90% by weight, preferably at least 95% byweight, especially preferably at least 98% by weight, monomeric unitsderived from caprolactam, having a tensile strength of at least 80cN/tex with a hot-air shrinkage of 1.0 to 6.5% in accordance with ASTMD4974-04 at 177° C. for 2 minutes.

A further solution to the problem is provided by a low-shrinkagealiphatic polyamide yarn, produced from a polyamide which has acomposition of at least 90% by weight, preferably at least 95% byweight, especially preferably at least 98% by weight, monomeric unitsderived from hexamethylenediamine and adipic acid (AH salt), or mixturesthereof, having a tensile strength of at least 80 cN/tex with a hot-airshrinkage of less than 4% in accordance with ASTM D4974-04 at 177° C.for 2 minutes.

Such a yarn has the advantage that it is very simple to produce by theprocess described above and has a tensile strength of at least 80cN/tex, preferably of at least 82 cN/tex, especially preferably of atleast 84 cN/tex.

The determination of the glass transition temperature and the meltingtemperature of the yarn composed of polyamide is effected in accordancewith ASTM D 3418-03. Both glass transition temperature and meltingtemperature are measured in a temperature scan at 10° C./min.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in greater detail with reference to theaccompanying drawing, wherein:

FIG. 1 is a diagrammatic view of an apparatus according to the inventionfor the production of a yarn.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a diagrammatic view of an embodiment of an apparatus forthe production of an endless yarn from a synthetic polyamide melt. Usinga spinning device 1, a multiplicity of filament strands 4 are extrudedat a temperature of 40° to 70° C. above the melting temperature of thepolyamide used. The spinning device 1 comprises a spinneret 2 by meansof which the individual filaments of the filament strand are formed. Thefilament strands 4 are fed from the spinneret 2 at a specific speed to aquench duct 3 and cooled to a temperature below the glass transitiontemperature of the yarn, for example below 50° C. in the case of PA6.The cooling is usually effected by blowing cooled quenching air over thefilament strands 4. The filament strands 4 are then combined to form ayarn 6 and provided with a spinning lubricant 5. The yarn 6 passes to apair of inlet rolls 7, which consists of a driven roll and a non-drivenidler roll. The pair of inlet rolls 7 determines the draw-off speed ofthe yarn 6 from the quench duct 3, which in this exemplary embodimentcan be 793 m/min. The difference between the spinning speed at thespinneret and the draw-off speed of the yarn from the quench duct givesthe degree of drawing of the filament strands 4 in the spinning process.The filament strands 4 are drawn off by the inlet roll 7 at a speedhigher than the spinning speed.

Following the pair of inlet rolls 7, the yarn 6 is guided over at leastfive pairs of rolls 8, 9, 10, 11, 12. The yarn 6 wraps several timesaround the pair of inlet rolls 7 and each of the pairs of rolls 8, 9,10, 11, 12.

The drawing of the yarn 6 is effected in at least three stages: firstlybetween the pair of inlet rolls 7 and the first pair of draw rolls 8,then further between the pairs of draw rolls 8 and 9, and 9 and 10. Thepairs of draw rolls 8, 9, 10 are preferably in the form of duo rolls;both rolls are driven and have substantially the same external diameter.This has the advantage that the stepwise heating of the yarn 6 takesplace relatively continuously over a relatively long time, which isadvantageous for the transformation of the molecule chains. The surfacesof the pairs of draw rolls 8, 9, 10 are heated by means of a heatingsystem 15 having three different heat outputs as described below forrespectively heating the pairs of draw rolls 8, 9, 10. The heatingsystem may be, for example, an electrical, steam-based or fluid-basedheating system, it being assumed hereinbelow that in the course ofpassing at least twice around the pairs of draw rolls the yarn 6 hasalso adopted the temperature of the pairs of draw rolls 8, 9, 10.

The region between the inlet roll 7 and the pair of draw rolls 8 formsthe first draw zone in which, in accordance with exemplary embodiment 1,the yarn undergoes an increase in speed from 793 m/min to 817 m/min,passes at least twice around the draw rolls 8 and is heated to atemperature of 70° C.

The second draw zone is formed by the pairs of draw rolls 8 and 9, theyarn 6 passing around the draw rolls 9 at a speed of 2368 m/min at atemperature of 155° C.

The third draw zone is formed by the pairs of draw rolls 9 and 10, theyarn 6 passing around the draw rolls 10 at a speed of 3409 m/min at atemperature of 210° C. In accordance with this exemplary embodiment thedrawing between the inlet roll 7 and the pair of draw rolls 10 iseffected by the factor 4.3. The pair of draw rolls 10 in the third drawzone is operated at a temperature of 5° C. to 20° C. below the meltingpoint of the yarn. Accordingly, the pair of draw rolls 10 is operated ata temperature of 200° C. to 215° C. in the case of a yarn composed ofPA6 and at a temperature of 240° C. to 255° C. in the case of a yarncomposed of PA6.6.

The drawing step is followed by an at least three-stage yarn relaxationstep, preferably by means of duo rolls, wherein the pairs of rolls 10and 11 form the first relaxation zone. The speed of the yarn 6 decreasesstepwise and falls from 3409 m/min to 3272 m/min, the yarn being kept ata temperature slightly below the melting point. In this exemplaryembodiment the temperature of the PA6 yarn is 215° C., which is 5° C.below the melting point. According to the invention, a temperature rangeof 5° C. to 8° C. below the melting point of the yarn 6 has proved to beespecially advantageous. In the case of a relatively long dwell time(relatively large distances between the pairs of rolls, relatively largeroll diameters, different yarn speeds with the same degree of drawing)for thermal transformation, the temperature range during relaxation canalso be in the range of 5° C. to 15° C. In order to obtain the desiredyarn quality, the temperature range during relaxation can be veryslightly closer to the melting point than the temperature range duringdrawing.

The second relaxation zone is formed by the pairs of rolls 11 and 12,the yarn 6 being guided around the pairs of rolls 12 at the lower speedof 3150 m/min. In this case too, a temperature slightly below themelting temperature of the yarn 6 is maintained, analogously to thefirst relaxation zone.

In this exemplary embodiment a third relaxation step takes place betweenthe pair of rolls 12 and the reel device 14, wherein the yarn, cooled,is wound up at a speed of 3100 m/min. The degree of relaxation betweenthe pair of draw rolls 10 and the reel device 14 is 9%.

Alternatively, the third relaxation step can also be effected with asixth pair of rolls (not shown) arranged between the pair of rolls 12and the reel device 14. The relaxation can be intensified by the reeldevice 14, by the reel device winding up the yarn 6 at a lower speedthan the speed at which the latter travels around the pair of rolls 12or around the last pair of rolls. Depending upon the desired yarnproperties, the reel device can, however, also be operated at the samespeed as or at a higher speed than the last pair of rolls. For example,the speed range of the reel device can be from 0.8% to 1.2% higher thanthe speed of the last pair of rolls. In a further embodiment, the reeldevice may be operated at a speed 1% to 3% lower than the last pair ofrolls.

According to the invention, the relaxation takes place in three stagesat slightly below the melting temperature of the yarn, with the resultthat the molecule chains of the polyamide stabilise.

The relaxation of the yarn is effected by means of pairs of heatedrolls, preferably duo rolls, in order to provide the yarn with a minimumdwell time for thermal treatment, during which the molecule chains ofthe polyamide are able to stabilise; the two rolls are driven and havesubstantially the same external diameter. The yarn 6 wraps severaltimes, at least twice, around each roll. Alternatively, the use of monorolls is possible, provided the dwell time does not fall below aspecific value. The term “mono rolls” is used to denote pairs of rollsconsisting of a driven roll having a large diameter and an idler rollhaving a small diameter that is carried along therewith.

In accordance with the exemplary embodiment using polyamide 6, in thecase of a yarn composed of PA6 the following peripheral speeds andtemperature values apply to the first to fifth pairs of draw rolls:

first pair of draw rolls 8: 817 m/min at 70° C.,

second pair of draw rolls 9: 2368 m/min at 155° C.,

third pair of draw rolls 10: 3409 m/min at 210° C.,

fourth pair of rolls 11: 3272 m/min at 215° C.,

fifth pair of rolls 12: 3150 m/min at 215° C.

Unlike the prior art, the total drawing of the yarn 6 is effectedsubstantially between the pair of inlet rolls 7 and the third pair ofdraw rolls 10, since the subsequent speed of the yarn 6 at a veryslightly higher temperature, slightly below the melting point,decreases, with the result that the molecule chains of the polyamidestabilise, leading to increased tenacity.

Following the last pair of rolls 12, the yarn 6 is fed via aninterlacing means 13 to a reel device 14 and from there is wound onto areel at a speed of, for example, 3100 m/min, that is to say at a lowerspeed than the speed at which the fifth pair of rolls 12 is operated.This also results in stabilisation and fixing of the crystallinity andthe orientation of the molecules. Accordingly, after the third pair ofdraw rolls 10 the yarn 6 is no longer subjected to drawing but travelsat a successively lower speed up to the reeling step.

Accordingly, in respect of the yarn 6 this results in a draw ratio of4.3 from the inlet roll 7 up to the third pair of draw rolls 10 and adegree of relaxation of 9% from the pair of draw rolls 10 up to the reeldevice 14. The degree of drawing between the pair of inlet rolls 7 andthe first pair of draw rolls 8 is 1.03.

It has been found that if PA6 undergoes an initial high degree ofdrawing between the pairs of draw rolls 8 and 9 with a rise intemperature from a temperature that is from 5° C. below to 30° C. abovethe glass transition temperature to a temperature of 130° C. to 180° C.,which is 40° C. below the melting temperature of PA6, and a further highdegree of drawing between the pairs of draw rolls 9 and 10 and a furtherrise in temperature to 210° C. with subsequent relaxation at atemperature slightly below the melting point, a yarn composed of theeconomical PA6 exhibits mechanical strength values otherwise achievedonly by a yarn composed of the higher value PA6.6. Preferably there is arise in temperature from 65° C. to 75° C. from the pair of draw rolls 8to 150° C. to 170° C. at the pair of draw rolls 9, with the result thatespecially good values are achievable.

Surprisingly, this finding can also be applied to PA6.6, that is to sayan increase in the temperatures at the pairs of draw rolls 8 and 9 froma temperature that is from 5° C. below to 30° C. above the glasstransition temperature of PA6.6 at the pair of draw rolls 8 up to atemperature of 170° C. to 220° C. at the pair of draw rolls 9.

Using the process according to the invention, consistently stable valuesfor hot-air shrinkage and tenacity are achieved even in the case ofPA6.6.

EXAMPLE

In the following experimental examples, the first column (prior art)describes a process and a yarn according to the prior art. Columns 1-3show the process according to the invention and a yarn according to theinvention thereby produced.

In the Example, the hot-air shrinkage was determined at a temperature of177° C. after 2 min holding time in accordance with ASTM D4974-04. It istherefore not comparable to testing methods carried out with a holdingtime of 4 hours, for example for use in tyre manufacture.

Test No. Prior art 1 2 3 Test parameter Polyamide type PA6 with arelative PA6 with a relative viscosity of 3.0 viscosity of 2.8 measuredin 90% measured in 90% HCOOH HCOOH Temperature at the spinneret in [°C.] 292 290 290 290 Inlet roll speed [m/min] 730 793 793 775 First drawroll speed [m/min] 759 817 817 798 temperature [° C.] 60 70 70 70 Seconddraw roll speed [m/min] 2245 2368 2368 2314 temperature [° C.] 155 155155 155 Third draw roll speed [m/min] 3286 3409 3409 3409 temperature [°C.] 195 210 210 210 Fourth draw roll speed [m/min] 3080 3272 3272 3272temperature [° C.] 170 215 215 215 Fifth draw roll speed [m/min] — 31503150 3150 temperature [° C.] — 215 215 215 Reel device speed [m/min]3045 3100 3130 3125 Reel tension [cN] 60 60 65 65 Laboratory tests Titre[d/tex] 481 482 482 472 Number of filaments 140 140 140 140 Elongationat break [%] 22.3 26.7 25.9 26.2 Tensile strength [cN/tex] 83.5 81.481.6 84.7 Hot-air shrinkage 177° C., 2 min [%] 8.9 3.7 4.4 4.6

In the prior art the polyamide is drawn off by the inlet roll 7 at aspeed of 730 m/min at a temperature of 60° C. and up to the third drawroll 10 is subjected to drawing at a speed of 3286 m/min at atemperature of 195° C., which corresponds to 4.5-fold drawing. Therelaxation is effected with a single drawing mechanism, the fourth drawroll, at a speed of 3080 m/min at a temperature of 170° C., and furtherto the reel device at a speed of 3045 m/min, which corresponds to atotal relaxation of −7.3%.

As a result, the elongation at break is, at 22.3%, significantly lowerthan in accordance with the lowest value according to the invention of25.9%, and the hot-air shrinkage is double the value at 8.9%, whereasthe values according to the invention lie between 3.7 and 4.6%.

According to the invention, after a first multi-stage drawing step,which is followed by an at least three-stage relaxation step, thedesired tenacity values for a PA6, PA6.6 or a comparable polyamide areachievable. The temperature in the last drawing stage is 5° C. to 20° C.slightly below the melting point of the yarn. During the at leastthree-stage relaxation step, the temperature is likewise kept just belowthe melting point of the yarn, namely 5° C. to 15° C., preferably 5° C.to 8° C.

It will be understood that the above description of the presentinvention is susceptible to various modifications, changes andadaptations, and that the same are intended to be comprehended withinthe meaning and range of equivalents of the appended claims.

1-25. (canceled)
 26. An apparatus for production of a low-shrinkagealiphatic polyamide yarn, comprising: a spinneret in which polyamide isextruded through to form filaments; a quenching unit following thespinneret to cool and combine the filaments to form at least one yarn; afirst pair of inlet rolls following the quenching unit for a firstdrawing of the yarn; a second multi-stage drawing apparatus followingthe first pair of inlet rolls, wherein the second multi-stage drawingapparatus comprises at least three pairs of heatable draw rolls; meansfor increasing the temperature of the pairs of draw rolls from atemperature of a first pair of the heatable draw rolls that is operatedat a temperature of 5° C. below to 30° C. above the glass transitiontemperature of the yarn, to a temperature of a second pair of theheatable draw rolls that operate at a temperature from 130° C., to 40°C. below a melting temperature of the yarn, to a temperature of a lastpair of the heatable draw rolls that is operated at a temperature of 5°C. to 20° C. below the melting point of the yarn; and an at leastthree-stage relaxation zone, following the second multi-stage drawingapparatus, in which the yarn is relaxed from 4% to 10% in a temperaturerange of 5° C. to 15° C. below the melting point of the yarn, whereinthe at least three-stage relaxation zone comprises at least first andsecond pairs of rolls and a reel device.
 27. The apparatus according toclaim 26, wherein the pairs of draw rolls are operated at a peripheralspeed of 400 m/min to 4000 m/min.
 28. The apparatus according to claim26, wherein the polyamide comprises at least 90% by weight, monomericunits derived from caprolactam, or monomeric units derived fromhexamethylenediamine and adipic acid (AH salt), or mixtures thereof.